1. Field of the Invention
The present invention relates to an image-capturing apparatus and a control method of the image-capturing apparatus, and particularly, to an image-capturing apparatus and a control method of the image-capturing apparatus that can suppress a flicker generated in a captured image signal.
2. Description of the Related Art
Unlike in a CCD image sensor, the exposure timing is different in each line in a CMOS image sensor used as an image sensor of an image-capturing apparatus such as a digital camera. Therefore, a horizontally striped contrast change is generated in an image signal of one frame captured under a light source (flickering light source), such as fluorescent light, in which the luminance periodically changes according to a power frequency (such as 50 Hz and 60 Hz). Specifically, the luminance of the light source changes at a 1/100 second period when the power frequency is 50 Hz, and the luminance of the light source changes at a 1/120 second period when the power frequency is 60 Hz. The contrast change moves with time in the perpendicular direction relative to the exposure lines of the image sensor. The periodical contrast in the perpendicular direction will be called a flicker. To suppress the flicker, there is a known method of capturing an image at a shutter speed of a positive multiple of the period of the luminance change of the light source. More specifically, the shutter speed of the image-capturing apparatus is set to 1/50 second or 1/100 second if the power frequency is 50 Hz, and the shutter speed is set to 1/60 second or 1/120 second if the power frequency is 60 Hz. Hereinafter, 1/(period of luminance change of light source) will be called a flicker rate.
Meanwhile, in a still image shooting mode of the image-capturing apparatus, a display device displays through images (or live view images) in a standby state, and the display device often functions as an electronic view finder (EVF). A release SW1 that is turned on during half-press of the shutter button and a release SW2 that is turned on during full-press are often arranged on the image-capturing apparatus. A shooting-preparation operation starts when the release SW1 is turned on, and an image-capturing operation starts when the release SW2 is turned on. The shooting-preparation operation includes an auto-focus detection (hereinafter, called “auto focus” or “AF”) process, and the image-capturing operation includes an image-capturing process and a saving process of a captured image to a recording medium.
A typical image-capturing apparatus has an automatic exposure control function using program diagrams that define in advance combinations of appropriate shooting conditions (aperture and shutter speed (and ISO sensitivity as necessary) according to the subject luminance. It is known that the program diagrams are prepared according to the usage, and the program diagrams are selectively used.
For example, Japanese Patent Laid-Open No. 9-083945 discloses a video camera that uses different program diagrams in a moving image mode and a still image mode. Specifically, a program diagram that changes the aperture and the gain (ISO sensitivity) to maintain the shutter speed of 1/60 second is used in the moving image mode. In the still image mode, a program diagram is used in which the shutter speed is set greater than that in the moving image mode, and the shutter speed is set to 1/100 second in a range of the subject luminance equivalent to indoor brightness to allow suppressing the flicker.
In a conventional image-capturing apparatus such as a digital camera, the aperture is controlled to prevent the aperture in still image capturing from opening wider than the aperture in AF processing in order to capture a still image of a focused subject. This is to prevent the subject focused by the AF processing from being out of focus by opening the aperture in the still image capturing, because the depth of field is shallower when the aperture is opened wider. Therefore, for example, the program diagram used in the AF processing that starts after the release SW1 is turned on is different from the program diagram used when the release SW1 is turned off in order to prevent the aperture in the still image capturing from opening wider than the aperture in the AF processing.
However, assuming a still image shooting mode for recording a moving image in a predetermined period just before the release SW2 is turned on in addition to a still image captured when the release SW2 is turned on, the following problem may occur.
In the still image shooting mode, a moving image captured in a period when the release SW2 is off and the release SW1 is on are recorded, and a shooting-preparation process including the AF processing is executed in the period when the release SW1 is on. As described, the shutter speed that can suppress the flicker may not be set when the program diagram suitable for the AF processing is used in the AF processing. The flicker is included in the moving image captured in the AF processing, and unsuitable images are recorded as the moving image.